Semiconductor technology based on the use of GaN, AlN, and compounds thereof as semiconductor material, currently suffer from a lack of large-sized bulk substrates of such materials.
The most widely known technique for producing a bulk GaN substrate consists of depositing the GaN on a substrate by a process known as hetero-epitaxy.
In order to produce a substrate having good crystal quality, the substrate material and the epitaxial monocrystal to be deposited thereon, must have little lattice mismatch. Additionally, the thermal expansion coefficient of both the substrate material and the epitaxial monocrystal must be relatively similar, because the high temperatures involved in hetero-epitaxy is known to sometimes cause dissociation and diffusion of elements of the substrate to the epitaxial layer. Thirdly, the substrate material must be mechanically and chemically stable at high temperatures so as to obtain good crystal quality. Naturally, qualities such as surface condition and crystal quality of the starting substrate are also important factors.
Currently, sapphire and silicon carbide (SiC), are commonly used as the substrate material. However, sapphire and silicon carbide are far from being optimal substrate material due to their lattice parameters and their expansion coefficients.
Another common practice is growing layers of GaN on substrates such as zirconium oxide (ZnO), lithium gallium oxide (LiGaO2), lithium aluminum oxide (LiAlO2) (See “Growth of III-Nitrides on ZnO, LiGaO2 and LiAlO2 substrates”, Mackenzie et al., J. Electrochem. Soc., vol. 145, No 7, July 1998, p. 2581) or of neodymium gallium oxide (NdGaO3) (see “GaN bulk substrates for GaN based LEDs and LDs”, Oda et al., Phys. Stat. Sol., (a) 180, 51 (2000). Although these substrate materials are often selected for their small lattice mismatch and their similar coefficient of expansion with GaN, they suffer from having poor chemical stability under high temperatures as compared to sapphire or silicon carbide. For example, when the oxide substrates are exposed to high temperatures, dissociation of the metal and/or the oxygen occurs and such dissociated metal and/or oxygen diffuses to the epitaxial layer. As described in Impurity contamination of GaN epitaxial films from the sapphire, SiC, and ZnO substrates, Popovici et al., Appl. Phys. Lett., 71 (23), 8 Dec. 1997, and incorporated herein, contamination of the epitaxial layer by zinc and oxygen from a ZnO substrate compromises the quality and purity of the epitaxial layer.
Furthermore, once the layer or layers intended to form the substrate have been formed, in the majority of cases the support on which growth has been carried out has to be removed, which necessitates either chemical attack of said support and thus its loss even if it is produced from an expensive material, thereby increasing the cost of the process, or by rupture between the layers formed by epitaxial growth and said support, which can be difficult to control and/or can necessitate particular dispositions which complicate the method or make it more expensive.
More generally, it has been proven that, for the purpose of temporary support removal, chip manufacturers mostly prefer an etching technique rather than a rupturing technique. This is mainly because, in the semiconductor industry, etching techniques have been mastered for years and most often do not require any additional investment while rupture techniques are more difficult to control or require significant capital investment, thus adding complexity to the process.
An additional technique for separating the support and the substrates generates stress at the boundary between the support and the substrate by means of laser illumination. Illuminating the support-substrate wafer can generate sufficient stress at the boundary to cause layer separation. Stress can be generated by phase transition, gas evolution, vaporization or sublimation. It is advantageous for one of the support or the substrate to be significantly transparent at the laser wavelength and the other to be significantly absorbent so that absorption of the laser radiation is concentrated at their boundary. See, e.g., U.S. Pat. No. 6,372,608 B1 (especially FIG. 23).
In parallel, an etching approach would be valuable only if the material of the support is relatively inexpensive, but such inexpensive materials, such as gallium Arsenide GaAs, introduce additional drawbacks. In this regard, the article “Preparation of large freestanding GaN substrates by Hydride Vapor Phase Epitaxy using GaAs as a starting substrate”, Motoki et al., Jpn. J. Appl. Phys., Vol. 41 (2001), p. 140-143, proposes using a gallium arsenide GaAs substrate on which to grow GaN by hetero-epitaxy. However, when heated to the high temperatures involved in epitaxy, GaAs undergoes surface dissociation, which causes arsenic to evaporate, which can contaminate the GaN monocrystal.
The present invention now seeks to overcome these disadvantages.